Field of the Invention
The present invention relates to a process for preparing an aqueous dispersion of particles composed of addition polymer and finely divided inorganic solid (composite particles), in which process a mixture of ethylenically unsaturated monomers is dispersely distributed in aqueous medium and is polymerized by the method of free-radical aqueous emulsion polymerization by means of at least one free-radical polymerization initiator in the presence of at least one dispersely distributed, finely divided inorganic solid and at least one dispersant. The present invention further relates to the use of the aqueous composite-particle dispersion and of the composite-particle powders.
Aqueous dispersions of composite particles are general knowledge. They are fluid systems whose disperse phase in the aqueous dispersion medium comprises polymer coils consisting of a plurality of intertwined polymer chainsxe2x80x94known as the polymer matrixxe2x80x94and particles composed of finely divided inorganic solid, which are in disperse distribution. The diameter of the composite particles is frequently within the range from 50 to 5,000 nm.
Like polymer solutions when the solvent is evaporated and aqueous polymer dispersions when the aqueous dispersion medium is evaporated, aqueous dispersions of composite particles have the potential to form modified polymer films containing finely divided inorganic solid, and on account of this potential they are of particular interest as modified bindersxe2x80x94for example, for paints or for compositions for coating leather, paper or plastics films. The composite-particle powders obtainable in principle from aqueous dispersions of composite particles are, furthermore, of interest as additives for plastics, as components for toner formulations, or as additives in electrophotographic applications.
The preparation of aqueous dispersions of composite particles is based on the following prior art.
A process for preparing polymer-enveloped inorganic particles by means of aqueous emulsion polymerization is disclosed in U.S. Pat. No. 3,544,500. In this process the inorganic particles are coated with water-insoluble polymers before the actual aqueous emulsion polymerization. The inorganic particles thus treated in a laborious process are dispersed in an aqueous medium using special stabilizers.
EP-A 104 498 relates to a process for preparing polymer-enveloped solids. A characteristic of the process is that finely divided solids having a minimal surface charge are dispersed in the aqueous polymerization medium by means of a nonionic protective colloid and the ethylenically unsaturated monomers added are polymerized by means of nonionic polymerization initiators.
U.S. Pat. No. 4,421,660 discloses a process for preparing aqueous dispersions whose disperse particles feature inorganic particles surrounded completely by a polymer shell. The aqueous dispersions are prepared by free-radically initiated aqueous emulsion polymerization of hydrophobic ethylenically unsaturated monomers in the presence of inorganic particles in disperse distribution.
A process for polymerizing ethylenically unsaturated monomers in the presence of uncharged inorganic solid particles stabilized in the aqueous reaction medium using nonionic dispersants is disclosed in U.S. Pat. No. 4,608,401.
The free-radically initiated aqueous emulsion polymerization of styrene in the presence of modified silicon dioxide particles is described by Furusawa et al. in Journal of Colloid and Interface Science 109 (1986) 69 to 76. The spherical silicon dioxide particles, having an average diameter of 190 nm, are modified using hydroxypropylcellulose.
Hergeth et al. (see Polymer 30 (1989) 254 to 258) describe the free-radically initiated aqueous emulsion polymerization of methyl methacrylate and, respectively, vinyl acetate in the presence of aggregated, finely divided quartz powder. The particle sizes of the aggregated quartz powder used are between 1 and 35 xcexcm.
GB-A 2 227 739 relates to a special emulsion polymerization process in which ethylenically unsaturated monomers are polymerized using ultrasound waves in the presence of dispersed inorganic powders which have cationic charges. The cationic charges of the dispersed solid particles are generated by treating the particles with cationic agents, preference being give to aluminum salts. The document, however, gives no details of particle sizes and stability of the aqueous dispersions of solids.
EP-A 505 230 discloses the free-radical aqueous emulsion polymerization of ethylenically unsaturated monomers in the presence of surface-modified silicon dioxide particles. They are functionalized using special acrylic esters containing silanol groups.
U.S. Pat. No. 4,981,882 relates to the preparation of composite particles by means of a special emulsion polymerization process. Essential features of the process are finely divided inorganic particles dispersed in the aqueous medium by means of basic dispersants; the treatment of these inorganic particles with ethylenically unsaturated carboxylic acids; and the addition of at least one amphiphilic component for the purpose of stabilizing the dispersion of solids during the emulsion polymerization. The finely divided inorganic particles preferably have a size of between 100 and 700 nm.
Haga et al. (cf. Angewandte Makromolekulare Chemie 189 (1991) 23 to 34) describe the influence of the nature and concentration of the monomers, the nature and concentration of the polymerization initiator, and the pH on the formation of polymers on particles of titanium dioxide dispersed in an aqueous medium. High encapsulation yields of the titanium dioxide particles are obtained if the polymer chains and the titanium dioxide particles have opposing charges. However, the publication contains no information on the particle size and the stability of the titanium dioxide dispersions.
In Tianjin Daxue Xuebao 4 (1991) pages 10 to 15, Long et al. describe the dispersant-free polymerization of methyl methacrylate in the presence of finely divided particles of silicon dioxide and, respectively, aluminum. High encapsulation yields of the inorganic particles are obtained if the end groups of the polymer chains and the inorganic particles have opposing charges.
EP-A 572 128 relates to a preparation process for composite particles in which the inorganic particles are treated with an organic polyacid or a salt thereof at a defined pH in an aqueous medium, and the subsequent free-radically initiated aqueous emulsion polymerization of ethylenically unsaturated monomers takes place at a pH less than 9.
Bourgeat-Lami et al. (cf. Angewandte Makromolekulare Chemie 242 (1996) 105 to 122) describe the reaction products obtainable by free-radical aqueous emulsion polymerization of ethyl acrylate in the presence of functionalized and unfunctionalized silicon dioxide particles. The polymerization experiments were generally carried out using anionically charged silicon dioxide particles, the nonionic nonylphenol ethoxylate NP30 and the anionic sodium dodecyl sulfate (SDS) as emulsifiers and potassium peroxodisulfate as free-radical polymerization initiator. The authors describe the resulting reaction products as aggregates containing more than one silicon dioxide particle or as polymer clusters which form on the silicon dioxide surface.
Paulke et al. (cf. Synthesis Studies of Paramagnetic Polystyrene Latex Particles in Scientific and Clinical Applications of Magnetic Carriers, pages 69 to 76, Plenum Press, New York, 1997) describe three fundamental synthesis routes for preparing aqueous polymer dispersions containing iron oxide. Because of the deficient stability of the aqueous dispersion of solids, the use of freshly precipitated iron(II/III) oxide hydrate is an unavoidable precondition for all of the synthesis routes. In the first synthesis route, in the presence of this freshly precipitated iron(II/III) oxide hydrate, the free-radically initiated aqueous emulsion polymerization of styrene takes place with SDS as emulsifier and potassium peroxodisulfate as polymerization initiator. In the authors"" favored second synthesis route, styrene and methacrylic acid are polymerized in the presence of the freshly precipitated iron(II/III) oxide hydrate, the emulsifier N-cetyl-N,N,N-trimethylammonium bromide (CTAB), and special surface-active polymerization initiators (PEGA 600) in methanolic/aqueous medium. The third synthesis route uses ethanol and methoxyethanol as polymerization medium, hydroxypropylcellulose as emulsifier, benzoyl peroxide as polymerization initiator, and a special iron(II/III) oxide/styrene mixture in order to prepare polymer dispersions containing iron oxide.
Japanese laid-open specification JP 11-209622 discloses a process for preparing core/shell particles having a silica particle core and a polymer shell. The core/shell particles are prepared by subjecting the silica particles, which are in colloidal form in an aqueous medium, to pretreatment with a cationic vinyl monomer or radical initiator, followed by radically initiated aqueous emulsion polymerization with ethylenically unsaturated monomers.
Armes et al. (cf. Advanced Materials 11 (5) (1999) 408 to 410) describe the preparation of silicon dioxide composite particles which are obtainable in an emulsifier-free, free-radically initiated aqueous emulsion polymerization at a pH of 10 with special olefinically unsaturated monomers in the presence of dispersed silicon dioxide particles. Postulated as a precondition for the formation of polymer particles containing silicon dioxide is a strong acid/base interaction between the polymer formed and the acidic silicon dioxide particles used. Polymer particles containing silicon dioxide were obtained with poly-4-vinylpyridine and copolymers of styrene and, respectively, methyl methacrylate with 4-vinylpyridine. From 4 to 10 mol % is mentioned as the lowest possible content of 4-vinylpyridine in methyl methacrylate or styrene monomer mixtures capable of forming composite particles containing silicon dioxide.
The German patent application bearing the file reference 19942777.1, unpublished at the priority date of the present specification, describes a process for preparing aqueous composite-particle dispersions wherein the dispersed inorganic solid particles and the radical-generating and/or dispersive components used in the free-radically initiated aqueous emulsion polymerization have opposite charges.
It is an object of the present invention to provide an improved process for preparing an aqueous dispersion of composite particles by the method of free-radically initiated aqueous emulsion polymerization which is unhampered or less hampered by the disadvantages of the known processes and provides composite particles having a balanced profile of good performance properties.
We have found that this object is achieved by a process for preparing an aqueous dispersion of composite particles composed of addition polymer and finely divided inorganic solid, in which process a mixture of ethylenically unsaturated monomers is dispersely distributed in aqueous medium and is polymerized by the method of free-radical aqueous emulsion polymerization by means of at least one free-radical polymerization initiator in the presence of at least one dispersely distributed, finely divided inorganic solid and at least one dispersant, wherein
a) a stable aqueous dispersion of said at least one inorganic solid is used, said dispersion having the characteristic features that at an initial solids concentration of xe2x89xa71% by weight, based on the aqueous dispersion of said at least one solid, it still contains in dispersed form one hour after its preparation more than 90% by weight of the originally dispersed solid and its dispersed solid particles have a weight-average diameter xe2x89xa6100 nm,
b) the dispersed particles of said at least one inorganic solid exhibit a nonzero electrophoretic mobility in an aqueous standard potassium chloride solution at a pH which corresponds to the pH of the aqueous reaction medium at the beginning of the emulsion polymerization, and
c) the mixture of ethylenically unsaturated monomers contains  greater than 0 and xe2x89xa64% by weight, based on its overall amount, of at least one ethylenically unsaturated monomer A, which comprises either
at least one acid group and/or its corresponding anion, if the dispersed particles of said at least one inorganic solid have an electrophoretic mobility with a positive sign under the abovementioned conditions, or
at least one amino, amido, ureido or N-heterocyclic group and/or its ammonium derivatives alkylated or protonated on the nitrogen, if the dispersed particles of said at least one inorganic solid have an electrophoretic mobility with a negative sign under the above-mentioned conditions.